The Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission is a CO2 sensing project that uses a lidar system. The requirement of the mission is to accurately map sources, sinks and concentrations of global CO2 levels across the globe. The lidar system relies on the ratio between light scattered from the ground at two slightly different wavelengths around the 1.57 μm is CO2 absorption line. One wavelength is tuned to the peak of this absorption and is referred to as “online”. The other is positioned slightly away from this spectral peak and is referred to as “offline”. Using the ratio of light scattered from the ground at these two wavelengths it is possible to obtain the column integrated CO2 within the atmosphere.
A continuous wave (CW) solid state laser concept has been developed for the ASCENDS mission. Its advantage over a pulsed laser system is an expected longer lifetime as it utilizes more reliable technology, such as that used in the telecommunications industry. However, a pulsed laser concept has an advantage that the effects of cloud scattering may be more easily separated from the ground scattered signal. For a CW system, light scattered from clouds represents noise in the CO2 retrieval, as the light passes through a smaller path length than that scattered from the ground. This may lead to an underestimate of the column density.
The present invention, as will be explained, includes a new signal processing concept for a CW lidar system that removes the cloud scatter noise signal. The present invention includes a chirp pulsed waveform in which the frequency sweep is stepped in time, rather than linearly varied in time.